Related Stories

The Moon is a major part of our lives. It is responsible for two-thirds of the height of the tides that wash across our beaches twice every day. It also stabilises the tilt of the Earth, thus making life possible.

In pre-industrial times, it gave us that very important light at night, and of course it's related strongly to the female menstrual cycle.

So if the Moon features so prominently in our lives, why do so many people wrongly believe that the Moon doesn't rotate? That is 100 per cent wrong.

But if you believe that you always see exactly the same face of the Moon, you're only nine per cent wrong.

The Moon takes about 27.3 days to perform one complete orbit around the Earth. And that's the same time that it takes for the Moon to perform one rotation on its spin axis. (Yes, that's right, the Moon does spin.)

It's easy to see why this myth-conception of the Moon not spinning appears perfectly reasonable. Do a simple experiment and plonk yourself in a rotating chair. (Think of yourself as the Earth.) Get a friend to walk around you in a circle, always keeping their face pointing at you while you spin around, following their path.

Your friend is the Moon.

To you, it's all quite straightforward. As they (the Moon) walk around you (the Earth sitting in the spinning chair), you can always see the face of the person. Therefore, as far as planet Earth is concerned, the Moon doesn't appear to be rotating.

But then get someone else to take your place in the spinning chair. Stand yourself outside the circle that your friend, the Moon, makes around the Earth.

As the Moon walks around the Earth, carefully always pointing their face at the Earth, you'll notice that you, the external observer, will first see their face, then one side of their head, then the back of their head, the other side of their head, and then finally the face.

You are standing still, so they have to be rotating. So the Moon does in fact rotate, and it does so in the same time that it takes to do one complete orbit around the Earth.

Physicists call this 'synchronous rotation'. It's caused by two separate factors — a thing called 'tidal force' and the fact that the denser parts of the Moon are on the side of the Moon facing the Earth.

But the synchronous rotation is not perfect, so the Moon does not always show exactly the same face to the Earth.

We can actually see slightly over half of the Moon's surface, about 59 per cent, if you observe it over a month. This wobbling effect is called 'libration', and comes from the Latin word, libra, meaning 'a balance'.

There are three separate librations. Each one lets us see slightly more than 50 per cent of the Moon.

Together, they add up to let us see an extra nine per cent of the Moon's surface.

First, diurnal means 'relating to the day'. You can a see slightly different view of the Moon at moonrise and moonset.

At moonrise, we can see slightly more of the east side of the Moon. But at moonset (I bet you've never said that word before), you can see slightly more of the west side of the Moon.

The second factor is that the Moon travels in an elliptical, or sort of a rugby ball-shaped, orbit around the Earth, rather than a perfect circular orbit.

This means that when it is close to the Earth it moves faster, and when it is further away it moves more slowly.

But the Moon always spins at the same rotational speed. So there is a mismatch.

So sometimes we can see a little around the leading edge of the Moon, and later we can see a little around the trailing edge of the Moon.

Finally, the Moon does not orbit around the Earth directly above our equator. No, the Moon's orbit is tilted by 6.7 degrees to the Earth's equator.

So sometimes we can see more of the Moon's south pole, and at other times, more of its north pole.

So we know that the Moon does rotate, and that it does not show exactly the same face to the Earth at all times.

But we still don't know why the full Moon looks so huge as it rises just above the horizon, even though you can measure it and see that it's not really bigger.